1. Field of the Invention
This invention relates to pumping apparatus for transporting fluids from a well formation to the earth's surface. More particularly, embodiments of the invention pertain to an improved electrical pump comprising a downhole linear electric motor and a positive displacement pump assembly. In addition, embodiments of the invention relate to the use of a plurality of submersible electrical pumps in the completion or operation of a well.
2. Description of the Related Art
Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting formation fluids to the earth's surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation, thereby inhibiting the unassisted flow of production fluid to the surface.
A common approach for urging production fluids to the surface includes the use of a mechanically actuated, positive displacement pump. Mechanically actuated pumps are sometimes referred to as “sucker rod” pumps. The reason is that reciprocal movement of the pump necessary for positive displacement is induced through reciprocal movement of a string of sucker rods above the pump from the surface.
A sucker rod pumping installation consists of a positive displacement pump disposed within the lower portion of the production tubing. The installation includes a piston which is moved in linear translation within the tubing by means of steel or fiberglass sucker rods. Linear movement of the sucker rods is typically imparted from the surface by a rocker-type structure. The rocker-type structure serves to-alternately raise and lower the sucker rods, thereby imparting reciprocating movement to the piston within the pump downhole.
Certain difficulties are experienced in connection with the use of sucker rods. The primary problem is rooted in the fact that most wells are not truly straight, but tend to deviate in various directions en route to the zone of production. This is particularly true with respect to wells which are directionally drilled. In this instance, deviation is intentional. Deviations in the direction of a downhole well cause friction to occur between the sucker rod joints and the production tubing. This, in turn, causes wear on the sucker rod and the tubing, necessitating the costly replacement of both. Further, the friction between the sucker rod and the tubing wastes energy and requires the use of higher capacity motors at the surface.
To overcome this problem, submersible electrical pumps have been developed. These pumps are installed into the well itself, typically at the lower end of the production tubing. State of the art submersible electrical pumps comprise a tubular assembly which resides at the base of the production string. The pump includes a rotary electric motor which turns turbines at a high horsepower. These turbines are placed below the producing zone of a well and act as fans for forcing production fluids upward through the wellbore.
Efforts have been made to develop a linear electric motor for use downhole. One example is U.S. Pat. No. 5,252,043, issued to Bolding, et al., entitled “Linear Motor-Pump Assembly and Method of Using Same.” Other examples include U.S. Pat. No. 4,687,054, issued in 1987 to Russell, et al. entitled “Linear Electric Motor For Downhole Use,” and U.S. Pat. No. 5,620,048, issued in 1997, and entitled “Oil-Well Installation Fitted With A Bottom-Well Electric Pump.” In these examples, the pump includes a linear electric motor having a series of windings which act upon an armature. The pump is powered by an electric cable extending from the surface to the bottom of the well, and residing in the annular space between the tubing and the casing. The power supply generates a magnetic field within the coils which, in turn, imparts an oscillating field upon the armature. In the case of a linear electric motor, the armature is translated in an up-and-down fashion within the well. The armature, in turn, imparts translational movement to the pump piston residing below the motor. The piston enables a positive displacement pump to displace fluids up the wellbore and to the surface with each stroke of the piston.
Submersible pump assemblies which utilize a linear electric motor have not been introduced to the oil field in commercially significant quantities. Such pumps would suffer from several challenges, if employed. A first problem relates to the introduction of the submersible pump into the wellbore. As noted, wellbores tend to have inherent deviations. At the same time, submersible pumps can be of such a length that it becomes difficult for the pump to negotiate turns and bends within the tubing string of the well. The length of a linear submersible pump is generally proportional to the horsepower desired to be generated by the pump assembly. Greater horsepower would be needed for deeper wells in order to overcome the prevailing hydrostatic head. This, in turn, would require a greater length or number of windings within the stator and corresponding armature.
Overriding this concern is the expense of manufacturing and stocking submersible pumps of various sizes. In this respect, the size of the electric motor is not standard, but is dependent upon the individual needs of each well and the amount of power, force and length of stroke desired.
Another problem relates to the inconsistent power sources at wellsites. Working a well necessarily involves the stopping and starting of the motor for more efficient production. Power surges associated with the start of the motor create harmful temperature variations and mechanical stresses which cause wear of the electrical insulators, connections and coils. Further, power sources themselves provide inconsistent electricity flow. Power spikes, interruptions in services, and other causes of uneven power supply generate, by the Joule effect, temperature variations that accelerate aging of electrical components. Considering that voltages acting upon the electrical components may range from 1000 volts to even 3000 volts, significant wear from inconsistent power presents a real source of wear. Hence, a system which provides for redundant electromagnetic coils within a stator for the submersible electrical pump is needed.
Also pertaining to the electrical system of a motor is the problem of line loss within the power cable. Current pumps utilize AC power directed from the surface to the motor. The use of AC power creates the potential for high power loss as electrical current is directed downward, caused by such factors as the inherent resistivities and resonant frequencies within the lines.
An additional problem encountered in submersible electrical pumps is the corrosive effect of the formation fluids themselves. Many rotary pump failures arise from short-circuits which take place in the electrical connection with the downhole motor. Such short-circuits are often due to normal progressive degradation of the electrical insulation barriers around the power cable. Those skilled in the art will appreciate that hydrocarbon wells are drilled for the purpose of exposing oil-bearing formations below an earth surface. Production fluids typically include water, hydrocarbons, acidic gases and other corrosive materials that invade the borehole during production. Such fluids attack the integrity of the electrical components, resulting in failure of the circuitry of the motor.
The circuit arrangement of the submersible pumps themselves exacerbates the problem. Submersible pump designs have been wired with coils or “windings,” in series. The result is that if one coil fails, power to the entire electrical assembly fails. Thus, a redundant system of coils, and even of pumps, is desirable.
Still another problem inherent in current submersible pump designs pertains to the restricted diameter for fluid flow within the motor section. In linear submersible pump designs, the motor portion of the pump is configured above the piston and sucker rod pump portion. The result is that fluid being displaced by the pump must travel through restrictive fluid ports which reside within the armature portion of the motor en route to the surface. Typically, the inner diameter of the production string defines an already narrow path of flow through which production fluids must travel. Positioning a linear electric motor within the tubing creates a further restriction for fluid movement. Therefore, a linear electrical pump design which provides for a hollow bore through the armature is desirable. Further, there is a need for such a design where the housing for the stator is in series with the production tubing, rather than residing within the production tubing. In this way, a larger armature and armature bore are provided.
When a submersible pump is in need of repair or replacement, most current pump designs require that the entire production string be pulled. This means that a workover unit capable of pulling string must be mobilized to the wellsite, oftentimes at remote locations. Further, the time incident to setting up and pulling the string requires a costly cessation of production operations. This challenge is particularly severe in the case of an offshore well.
Pulling the tubing is made more difficult and time consuming because the power cable to the downhole electric motor is tied to the outside of the production tubing. Hence, the cable must be disconnected from the tubing and otherwise manipulated as the tubing string is pulled. Thus, a linear electrical pump design having valves which are wireline retrievable is also needed.
In view of these challenges, it is apparent that an improved submersible electrical pump is desired. In addition, a method of completing a well utilizing a plurality of submersible electrical pumps is needed. In this manner, backup pumps are available in the event one pump fails, or in the event additional pumping capacity is needed downhole.